Soybeans reaching critical stage for fungicide
Jul 2, 2008 10:47 AM,
By Trey Koger
Extension Soybean Specialist
Mississippi State University
With much of Mississippi’s April-planted full-season soybean crop approaching the critical stage for fungicide applications, it’s time to discuss the factors that determine when to apply a fungicide.
With much of Mississippi’s April-planted full-season soybean crop approaching the critical stage for fungicide applications, it’s time to discuss the factors that determine when to apply a fungicide.
As of late June, we have found no soybean rust in Mississippi. The hot and dry conditions most of us are experiencing are not favorable for soybean rust development and spread.
In the absence of soybean rust, application timing for a fungicide is based on these factors.
Irrigated or non-irrigated acres: A timely application of a strobilurin fungicide (Quadris or Headline) is recommended on irrigated soybean acres. The return on applying a fungicide is likely to be greater for a monoculture soybean system or a soybean-rice rotation.
However, even though the return may not be as great, the return is still positive when soybean is grown in rotation with corn or cotton, especially on irrigated acres. This is especially true this year with high soybean prices and the greater potential for a higher return on your investment.
On non-irrigated acres, the decision to apply a fungicide depends more on weather conditions at the critical time of reproductive development of the plant.
If hot, dry weather conditions continue, we are less likely to see an economic return from a fungicide application on non-irrigated acres. One thing to keep in mind: this crop is quite a bit later than those of the past several years and the weather conditions can change quickly, resulting in conditions favorable for disease development.
The decision to apply a fungicide on non-irrigated acres should be based on the condition of the crop and the weather conditions when the crop reaches the R3 to R4 reproductive growth stages.
Plant development: A foliar fungicide application should be applied to plants that have as much vegetative growth as possible when the plants are in R3 to R4 reproductive growth stages.
This year’s crop is different from past years in that soybean plants are reaching reproductive growth stages with less vegetative growth due to later planting and dry conditions.
It is common for an indeterminate variety to be at R3 one week and revert back to R2 to R3 the next week and then back to R3 the following week. This cycle can occur several times, especially with timely rains or irrigation.
It is better to apply a fungicide to a soybean plant with 16 nodes at the R3 reproductive growth stage than a plant with 14 nodes in the R3 growth stage. The more vegetation present at the time of a fungicide application, the more protected the plant is from foliar diseases.
Vegetation that develops after fungicide application is not protected from disease development.
Weather conditions: The decision to apply a fungicide should be based on weather conditions at the time of R3 to R4 stage of development. Holding off applying a fungicide application for a week is advised under hot and dry conditions. This allows the plant to produce more vegetative growth that can be protected and to get more residual activity from the fungicide longer into the season.
If weather conditions are cooler and wetter, disease development would be more prolific, and we would recommend applying fungicides when the plant reached the initial R3 stage of development.
Insect thresholds: A lot of fungicides are applied with an insecticide, usually a pyrethroid. To date, insect pressure has been relatively light, but we are beginning to see stink bug and bean leaf beetle numbers build, and more immature stink bugs and stink bug egg masses.
Where insect numbers have not reached threshold numbers and an insecticide is going to be applied along with the fungicide, the fungicide plus insecticide tank mix can be delayed in order to allow the plant to develop more vegetative growth and get closer to the R4 growth stage.
Data developed by Dan Poston of the Delta Research and Extension Center at Stoneville, Miss., has shown that in the absence of soybean rust, application of a strobilurin fungicide such as Quadris or Headline, at the R3 to R4 growth stage, provides the greatest yield increase and most consistent return on investment.
Product selection:Quadris and Headline can be interchanged with one another with only slight differences in activity on our common soybean diseases. Quadris is the preferred product in a soybean/rice rotation due to its excellent activity on aerial web blight. Headline is the preferred product in a continuous soybean, or soybean/corn, soybean/cotton rotation, or for a soybean variety that is extremely susceptible to frogeye leaf spot.
Use rates for both products are 4 to 6 fluid ounces per acre. Six ounces is recommended on irrigated acres, 4 ounces will likely be sufficient on dryland acres if hot and dry weather continues. Higher rates may perform better when cool and wet conditions are prevalent.
Higher rates will also reduce the likelihood of developing disease resistance to fungicides.
Applying Dimilin with Quadris or Headline will be a good option for late-planted soybeans that may encounter some late-season worm pressure, especially loopers. Dimilin provides control of grasshoppers and has been shown to provide some suppression of frogeye leaf spot.
Use rates for Quadris or Headline are 4 to 6 fluid ounces per acre plus Dimilin at 2 fluid ounces per acre. Use the same rate recommendations for Quadris and Headline described above.
If Quadris or Headline supplies are short and you need to stretch the supply of the two products over more acres, Topsin can be applied with either Quadris or Headline so that rates of Quadris or Headline can be reduced. Topsin has good activity on frogeye leaf spot, brown spot, and pod and stem blight, but should not be used as a standalone product.
Use rates for Quadris or Headline are 3 fluid ounces per acre plus 14 fluid ounces per acre of Topsin 4.5FL.
Both Quilt and Stratego contain less strobilurin fungicide than is recommended or applied when using a strobilurin-only fungicide. These two products can be used if Quadris or Headline are not available. Use rate for Quilt is 14 fluid ounces per acre (contains 4 ounces per acre Quadris). Use rate for Stratego is 10 fluid ounces per acre (contains 2.5 fluid ounces per acre of Gem).
Domark is our best-performing triazole-alone fungicide. It has both preventive and curative activity, meaning that it will control diseases present at the time of application as well as provide residual activity on diseases not yet infecting the plant.
Strobilurin fungicides (Quadris and Headline) provide only preventive disease control (disease that has not yet infected the plant).
Domark has similar activity as Quadris and Headline on several of our more problematic foliar diseases. Domark has excellent activity on frogeye leaf spot. Use rate is 4 to 5 fluid ounces per acre.
